Urinary preparations of nonprimate and human gonadotropins used for follicular development in macaques elicit neutralizing antibodies and ovarian refractoriness after 1-2 protocols. Follicular development over 3 successive stimulations with recombinant (r-) human gonadotropins was examined. Rhesus monkeys received a 7-day pretreatment with a GnRH antagonist (Antide; Ares Serono) which continued during administration of either r-human follicle stimulating hormone alone (r-hFSH; Ares Serono; n=6) or sequential treatment with r-hFSH followed by r-hFSH + r-human luteinizing hormone (r-Plan A; Ares Serono; n=6) for 3 stimulation cycles (C-1,2,3). When follicles r4 mm diameter were observed, r-human chorionic gonadotropin (r-hCG; Ares Serono) was given the next day. The total number of oocytes aspirated/animal 27 hr post-hCG did not differ between cycles within treatments nor within cycles between treatments. The percentage of mature oocytes collected at metaphase I + metaphase II was similar between C-1 (90%; 82%) and C-3 (88%; 83%) within r-hFSH and r-Plan A, respectively, and within cycles between treatments. Fertilization rates were comparable between C-1 (41 q 6%) and C-3 (37 q 16%) in r-hFSH, but increased from C-1 (31 q 14%) to C-3 (72 q 4%) in r-Plan A. Fertilization rates in C-1 were similar between groups, but were less in r-hFSH than r-Plan A following C-3. Circulating antibodies to hFSH and hCG were absent following C-1 in both groups. At the start of C-3, only one animal (r-hFSH) was FSH antibody-positive, while 2/5 (r-hFSH) and 1/5 (r-Plan A) were hCG antibody-positive. After C-3, 2/5 (r-hFSH) and 4/5 (r-Plan A) animals displayed FSH antibodies. In r-hFSH and r-Plan A, respectively, 2/3 and 3/4 antibody-positive animals yielded oocytes for insemination. Thus, 3 follicular stimulation cycles per macaque can be achieved with recombinant human gonadotropins. Although the total number of animals yielding fertilizable oocytes may decline, mature oocytes can undergo in vitro fertilization following repeated gonadotropin stimulation. PART ll, SECTION A SCIENTIFIC SUBPROJECT FORM